I've been fascinated by word pronunciations where changing the spelling of one syllable doesn't change its pronunciation, but rather changes another syllable in the word. The only two examples I can think of have very similar structures... the same vowels in both sets, where the changing of the second syllable vowel to either "a" or "e" changes how the "o" is pronounced in the first syllable.
The examples are:
Woman and Women
Modal and Model
Are there other examples of this? What are some possible reasons why the spelling is different for the unchanged syllable?